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This KIP proposes changing the ISR expansion logic on the leader and and the ISR validation logic on the controller to avoid bringing back fenced or shutting down replicas in the ISR. The leader will consider only unfenced replicas to be eligible to join the ISR. It will rely on the metadata cache to get this information via the metadata log. As the metadata cache is eventually consistent, the leader might try to add a replica - which was just removed by the controller - back to the ISR because it does not know that the replica was fenced by the controller yet. In order to avoid this, the controller will validate the new ISR and reject any AlterPartition request containing an ineligible replica with the newly introduced INELIGIBLE_REPLICA error code. For backward compatibility, OPERATION_NOT_ATTEMPTED will be used for older versions. When the leader receives an INELIGIBLE_REPLICA error code, it is expected to revert back its state to the last committed state - assuming that the state did not change in the mean time - and to retry to expansion. When a broker is unfenced by the controller, the leader does nothing because subsequent fetch requests from the followers will try to get them back into the ISR if they are caught-up.

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